This invention relates to wireless communication, and in particular, to a method and apparatus to select the data rate of a wireless network link according to a measure of error vector magnitude at a receiver.
A wireless network such as a wireless local area network (WLAN), includes a number of nodes each containing a wireless transceiver and modulator/demodulator (a “radio”). A wireless network may conform to one or more standards, e.g., one or more variants of the IEEE 802.11 standard such as IEEE 802.11a, 802.11b, or 802.11g. In an ad-hoc network, called an independent basic service set (IBSS), any nodes acts as station (STA) and communication is from STA to STA, while in an infrastructure network, a radio may be a (client) STA or an access point (AP), and all communication is via the AP.
Many wireless network standards permit a plurality of data link rates, each data rate typically having one or more modulation schemes and coding rates. A radio (AP or STA) needs to select a data link rate from those available, for example in order to optimize traffic throughput in the WLAN. For each of the data rates, the quality of a signal received at a receiver, e.g., the received signal-to-noise ratio or received carrier-to-noise ratio, needs to be above a particular level in order to achieve a “good-enough” level of performance, e.g., a low enough bit error rate or packet error rate. Moreover, the wireless link, i.e., the wireless channel from one radio to another, varies over time. Therefore, in order for the throughput to be optimum at all times, there is a need for radios not only to select the best initial data rate to use, but also to vary the data rate over time as conditions change.
One method for selecting the data rate is to consider the received signal strength indication—the RSSI—of signals received at a receiving radio from another radio. The RSSI, however, is not in itself a good indicator of signal quality. For example, a high RSSI signal in a strong multipath environment, such as in many indoor environments may in fact be signal of relatively low quality, albeit of high signal strength.
Another rate selection method is to iteratively determine the rate by starting with the an initial rate, which may be the highest rate, or may be some middle rate based on history, and retrying until a satisfactory rate is achieved. The iterative method includes the one radio starting transmitting to the other at the highest available rate, and if that initial rate does not lead to a response from the other radio indicating satisfactory reception, retrying at a reduced rate. The retries are repeated until a response is received indicating successful reception at the other radio. A variation that can be used with radios that have configurable transmit antennas, e.g., an array of switched antennas, or smart antennas, retries at the same rate but using a different antenna configuration. The rate is lowered when no antenna configuration provides good enough communication at a given rate. A history of attempts typically is maintained for this purpose.
Yet another method combines the iterative approach with requesting all potential interferers not to transmit. For example, in an IEEE 802.11 conforming network, a request-to-transmit (RTS) packet may be transmitted by the radio. The RTS is sent at the lowest rate and informs all other radios that the RTS-transmitting radio is about to transmit. A radio receiving the RTS does not transmit for a period of time.
Yet another method combines the iterative method and the RSSI-based method.
Any iterative method takes time to achieve the appropriate rate. An iterative method starting with the highest rate may take as many as 24 attempts before deciding to give up. Each additional attempt reduces the throughput in the wireless network.
There thus is a need to provide an improved method for a radio to select the data link rate for transmitting to a remote radio.